Preventing mobilization of trace metals in subsurface aquifers due to the introduction of oxygenated water

ABSTRACT

A method for providing the recharge of water into underground aquifers while preventing the mobilization of trace metals. The recharge water may be used for storage and subsequent withdrawal, or to regain or increase the long term beneficial use of an aquifer. The recharge water may be also be used to influence the groundwater flow in the aquifer. Water is treated and recharged by the addition of a small amount of a sulfide compound to remove dissolved oxygen and prevent dissolution of negative valence sulfur bearing minerals, such as pyrite, in the subsurface. The recharged water may increase the pressure head in the aquifer, alter the groundwater flow pattern to prevent the encroachment of objectionable quality water, or to segregate water of different quality. The recharge water may be fresh or brackish depending on the specific objectives of the application.

BACKGROUND OF THE INVENTION

This application is directed towards a method for preventing the mobilization of trace metals (such as arsenic) in subsurface aquifers by resisting the dissolution of negative valence sulfur compounds, such as pyrite, and by removing dissolved oxygen from water introduced into the subsurface. In particular, this method is designed to allow IS the use of underground aquifers for water storage without mobilizing trace metals such as arsenic above regulatory levels. The types of storage envisioned include Aquifer Storage and Recovery (ASR), Aquifer Recharge, or other aquifer management programs designed to introduce or inject water into subsurface aquifers for storage, recovery, and water level (pressure) maintenance at the point of introduction or injection or at points located some distance from the introduction or injection point. The introduction of water used for pressure maintenance may be related to increasing the hydraulic head in the aquifer for a variety of other purposes including replenishing the volume of water previously withdrawn from an aquifer, or altering the groundwater flow patterns to impede the progress of contaminants such as salt water or man made contaminants without causing additional contamination problem due to mobilization of trace metals; altering the groundwater flow pattern to segregate water of differing quality; or diverting groundwater to flow in different directions, The methods described herein also are useful for aquifer recharge wherein water is introduced to increase water levels in an aquifer so that the water may be available at other sites where users may desire to produce from the aquifer or where recharge is used to prevent land subsidence. The water is prepared for introduction, injection or recharge by the addition of a small amount of sulfide compound to resist trace metal mobilization such as arsenic and remove oxygen from water introduced into the subsurface.

Currently there are numerous systems for underground natural water storage and recharge. These include those method provided in publicly available disclosures.

U.S. Pat. No. 7,192,218 describes an underground porosity water storage reservoir that minimizes the impacts on surface uses of the reservoir site. There is no discussion related to the prevention of leaching of unacceptable trace metals.

U.S. Pat. No. 4,254,831 describes a method and apparatus for restoring or maintaining an underground aquifer that is plagued with decreased water flow due to an accumulation of undesirable, flow-impeding, agents in the aquifer. A series of injection wells are disclosed. Again, there is no discussion related to the prevention of mobilizing or leaching of unacceptable trace metals.

U.S. Pat. No. 7,138,060 discloses a method of in situ treatment of contaminated groundwater which includes identifying a site contaminated with a pollutant susceptible to degradation by sulfate reducing microorganisms. An amount of sulfate needed to metabolize the contaminants is estimated and applied. The sulfate concentration in solution is 1,000 ppm or more. This is a much higher concentration than would be acceptable for drinkable water or needed for the control of dissolved oxygen. The sulfate ion does not react with the dissolved oxygen as would a sulfide compound described herein.

The above mentioned patents do not disclose methods that provide for the control of oxygen in the water introduced or injected into a subsurface aquifer or the prevention of the dissolution of minerals in the subsurface that contain negative valence sulfides, such as pyrite, which contains arsenic. Lowering or eliminating the amount of dissolved oxygen will avoid trace metals, such as arsenic, from being mobilized into the underground water where such trace metals naturally exist in the natural underground aquifer rock material, i.e. strata.

The preservation and management of water resources has become an important focus of the environmental movement. In a great portion the United States and many other areas of the world, water is abundant only during seasonal periods or is impacted by encroachment of water degraded by salt or other contaminants. During wet periods, there are often excess amounts of water, which is lost because it cannot be economically stored for use later during dry periods or periods of drought. There are also often alternative sources of water available for recharge into an aquifer. These sources of water can be used to recharge the aquifer to alter the hydraulic head in the aquifer and alter groundwater flow patterns to segregate water of different quality in the aquifer.

In recent years, newer technology has been developed that allows water to be captured and stored in ways that are more economical than traditional storage methods. The new technology involves capturing excess water and recharging it to underground aquifers in certain subsurface geologic formations to increase the hydraulic head. The recharged water can be used to increase the water stored in the aquifer or alter the direction of groundwater flow to segregate water of differing quality. The process is commonly called artificial recharge (AR) or managed aquifer recharge (MAR), and it is conducted using wells for injection or basins, trenches, or infiltration galleries to introduce water to the aquifer either directly from the recharge structure or through overlying unsaturated materials commonly known as the vadose zone. Recharge can also occur into permeable zones below the aquifer of interest with an intent of influencing the hydraulic head of the aquifer through vertical migration of the recharged water or pressure wave from the underlying recharge zone.

MAR is a now proven technology for storing large volumes of water. There are sites where more than a billion gallons of water are stored annually by this process. Some common users of this technology are municipal water utilities, agriculture, and industry.

Recharging water into the ground is also done to recharge aquifer systems that are experiencing depletion due to over pumping. Injection of water into aquifers is done to replenish aquifer systems for both environmental and human benefits. Protection of the quality of underground water resources is also an issue of extreme concern; and, therefore, regulations have been developed to control underground injection so that underground water resources can be protected. In the United States the agency that regulates underground injection of water is the United States Environmental Protection Agency (EPA). Recharge water is typically fresh water but can be brackish water under certain circumstances.

The EPA has adopted a water quality standard for potable water with regard to arsenic of 10 parts per billion (ppb) and other trace metals. In certain areas, arsenic mobilization in the subsurface affects the process of introducing or injecting water into the ground for storage and recharge. The regulations require that water may not be introduced into an underground source of drinking water (USDW) if the act of injection causes the USDW to exceed a primary drinking water standard. It has been found in many cases that the recharge of waters for the purpose of storage or recharge causes arsenic (and potentially other metals) within the aquifer to exceed drinking water standards. This is a violation of the EPA rules and therefore, where trace metals such as arsenic are released above regulatory limits, the practice of injection for storage or recharge must cease.

In most cases, where introduction or injection of water has caused a violation of the drinking water standard for arsenic, it is due to the oxidizing components of the introduced or injected water reacting with the natural minerals in the geologic formation and leaching or dissolving arsenic from its native state as a solid. Arsenic is often a trace constituent that occurs in pyrite in the subsurface. Studies have shown that the mobilization of arsenic is due, in part, to reactions with pyrite by such oxidizing constituents as dissolved oxygen, nitrates, and disinfectants such as the hypochlorite ion, chlorine, ozone that are present in the introduced or injected water. The oxidants react with subsurface minerals, such as pyrite, which are native to the subsurface environment. For pyrite, arsenic is the trace metal most commonly released in this process.

The present disclosure involves a unique method to treat the water to remove dissolved oxygen and other oxidants and thereby prevent the dissolution of subsurface negative valence subsurface sulfur based minerals, such as pyrite.

The method of the present application relates to the unique chemical properties of the sulfide ion-bisulfide ion-hydrogen sulfide chemical species, in low concentrations, which are added to a water stream to remove dissolved oxygen, chlorine, and nitrogen oxides in the water prior to entry into a natural aquifer. The treated water also inhibits the dissolution of pyrite and similarly negative valence sulfur bearing minerals in subsurface geologic formations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an illustration of the sulfide injection control system and pH control system of the process described in this specification.

FIG. 2 shows a simplified illustration of the underground aquifer and pumping system as disclosed herein. The injection zone may be above, within, or below the aquifer of interest.

FIG. 3 illustrates examples of subsurface infiltration technologies which may employ the presently disclosed method. The basin geometry can be altered to form a trench, dry well, or infiltration gallery above or below the top of the zone of saturation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Leaching of arsenic and other trace metals into waters that have been recharged into the subsurface can be reduced by removing the dissolved oxidants from the introduced or injected water. Currently the methods being used and investigated to remove oxidants from water prior to introduction to the subsurface for storage and recharge involve relatively expensive methods such as nitrogen purging, membrane separation, and catalytic processes, which typically involve relatively expensive mechanical equipment. The method disclosed therein entails a new process that involves only the addition of a chemical (a sulfide compound) to the flow stream, which will react with and eliminate the dissolved oxidants. Another feature of the process is that addition of the injected sulfide compound(s) provides chemical resistance to the dissolution of subsurface minerals such as pyrite; and, therefore, prevent the release of arsenic and other trace metals in the subsurface waters. In order to chemically deoxygenate water to be placed into the subsurface for storage or recharge, a chemical reducing agent will react with the oxygen in solution and oppose the dissolution of minerals such as pyrite within the geologic formation.

Other methods of removal of the oxygen, disinfectants, and nitrogen oxides from solution prevents the direct oxidation of pyrite. Nevertheless, some amount of pyrite dissolution would still occur in order to re-establish equilibrium as predicted by Le Chatelier's Principle. However, the addition of the sulfide ion will drive the reaction backwards toward the precipitation of pyrite and therefore add additional resistance against pyrite dissolution as indicated by the following chemical equation.

4FeS₂+4 H2O→4Fe²⁺+7S²⁻+SO₄ ²⁻+8H⁺  (1)

It is the ability of the sulfide ion to de-oxygenate the water and also reverse the dissolution reaction of pyrite that is of fundamental importance in the disclosed process.

The basic oxidation reduction reactions related to the sulfide ion and hydrogen sulfide chemistry relevant to the present disclosure include:

HS⁻+4H₂O→SO₄ ⁻²+9H⁺+8e⁻  (2)

FeS₂+2H⁺+2e⁻→Fe⁺²+2 HS⁻  (3)

4ClO⁻+HS⁻→SO₄ ⁻²+4Cl⁻+H⁺  (4)

NO₃ ⁻+H₂O+HS⁻→SO₄ ⁻²+NH₃   (5)

The use of the sulfide ion at low concentrations provides the recharged water with reducing properties that are similar to those of the natural formation waters without adding additional chemicals that are not normally found in subsurface waters. This is important since the water industry and household users are already familiar with the technologies required to rapidly remove sulfides from ground water and prepare the water for potable use.

In practice, the present method will be utilized by mixing a sulfide compound such as sodium hydrosulfide into the water to be recharged/stored prior to its introduction or injection underground. Standard chemical mixing equipment is used for the mixing process.

When estimating the sulfide requirements of the present method, the following information is required given that sodium hydrosulfide, NaSH, is the source of the sulfide or hydrosulfide ion.

-   1) C (O,ppm), the dissolved oxygen concentration in ppm, -   2) F(NaSH), the concentrated NaSH given as the fraction of NaSH in     solution by weight.     The density of the NaSH solution can be estimated using one of the     following equations:

D(kg/l)=0.6404 F(NaSH)+0.9984

Where:

D(kg/l)=the concentrated NaSH solution density at 75° F.

The weight of sulfide species in the concentrated solution is given by the following equation:

Wt  sulfide  in  milligrams  (Wt(Smg))  in  1  liter = Wt(Smg) $\begin{matrix} {{{Wt}({Smg})} = {1\; E\; 06 \times {32/56} \times {F({NaSH})} \times {D\left( {{kg}\text{/}l} \right)}}} \\ {= {571,430\mspace{14mu} {F({NaSH})} \times \left\lbrack {{0.6404\mspace{14mu} {F({NaSH})}} + 0.9984} \right\rbrack}} \end{matrix}$

The volume of the concentrated NaSH solution, V(Smg), that needs to be added to a gallon of water to yield a 1 ppm solution of Sulfides is calculated using the following equation:

V(Smg)=[1 gal×1 mg/l]/Smg Gal NaSH/Gal ASR influent

-   Where V(Smg)=volume of NaSH required to be added to obtain 1 mg/l as     sulfides per gal of water being injected. -   Let V(NaSH, I MGD) equal the volume of a NaSH solution required to     add to 695 gal to obtain 1 ppm sulfides

V(1MGD)=695×1440×V(Smg)

For a 45% solution of NaSH,

-   V(Smg)=0.3.029 E-06 gal NaSH/Gal of injected water, -   V(NaSH, 1MGD)=3.03 gal

For a value of C ppm oxygen in the water to be injected, a minimum value of C/2 ppm of sulfide need to be added to remove the indicated oxygen concentration. The volume of NaSH that needs to be added is:C/2 V(Smg).

For a solution containing 8 ppm, then the volume of 45% NaSH solution required per day is:

C/2V(Smg)=8/2*3.03=12.12 gal/day.

Although the chemical compound sodium hydrosulfide is stated above, the present Method pertains to all sulfide compounds that cause the same reaction when mixed into water and recharged into the subsurface. Such compounds include any soluble Group Ia or Group 2a metal sulfides, hydrosulfides (bisulfides), or hydrogen sulfide gas. Group I and Group II metals are those identified on a standard chemical periodic chart.

According to the present disclosure, it is expected that a range for the amount of addition of the sulfide containing compound into the water entering the aquifer is 0.5-10 parts per million (ppm) by weight of sulfides, depending upon the water being treated. In another embodiment, a more typical maximum amount would likely be 6 ppm sulfides, and average amounts may commonly be as low as 3 or 4 ppm. The treatment amount would ultimately depend upon the concentration of those chemical species capable of reacting with the sulfides in the water that is recharged into underground storage.

In practice, the equipment used would comprise pumping or piping equipment commonly used to move water into underground storage. The pumping/metering system for injecting the sulfide compound would consist of suitable pumps, piping and storage equipment, such as drums or tanks, as well as regulating equipment that would match the volume of inlet water in the correct ratio.

The addition of the sulfide compound will normally raise the pH of the water slightly to approximately 8-8.5. It may possibly be raised to as much as 9 if the water is very pure with little buffering. Consequently, it may be desirable that the pH be brought back to the normal level of 7 by adding an acid. Suitable acids include HCl and dilute sulfuric acid. Additionally, CO2 addition is another embodiment to lower the pH which works by creating carboxylic acid. A typical addition amount could be approximately 0.6 to 1 lbs of CO2 per 1,000 gallons of water.

Since the addition of sulfides is likely to raise the pH of the solution, there is some potential that CaCO3 may precipitate at the point of sulfide introduction or injection into the water to be introduced or injected. In order to avoid problems, it may be desirable to add the acid into the water near the introduction or injection point for the sulfide solution, but not use a common introduction or injection point. It is important to prevent the release of hydrogen sulfide from the water prior to introduction or injection by not allowing the acid and sulfide compound to directly mix prior to addition into the water being treated for introduction or injection.

The effectiveness of sulfide compound is not reduced by turbidity but can be impacted if the iron concentration in solution exceeds regulatory standards. The level of sulfide in the water is low enough that no special precaution is required to protect the piping. Though sulfide stress cracking is a known problem for mild steel piping systems, common piping materials used for water such as PVC or fiberglass, can be employed to eliminate corrosion as an issue. The piping and pump materials used for the sulfide compound addition system should be designed to handle the sulfide compound which tends to have corrosive properties. Piping systems and materials resistive to the corrosive properties of the sulfide compound should be employed for success.

When withdrawing water from the aquifer, the sulfide compound is typically at a low levels.

If there is residual sulfide compound in the water being withdrawn from the aquifer it may be removed relatively easily. To remove the sulfide compound, a standard aeration system is used, such as sprays, flow over rocks, waterfall, etc. The sulfide reacts quickly with oxygen to become the sulfate ion.

The amount of sulfide to be added depends upon the water quality being introduced or injected into the underground aquifer. The water being extracted from the aquifer may also be monitored for oxygen and arsenic and this may be used to modify the amount of sulfide being used if needed on additional cycles.

FIG. 1 is a block diagram of a sulfide compound addition system and the pH control system. A sulfide compound, such as sodium hydrogen sulfide (or others as described above), is stored as an aqueous solution in tanks or drums 101. It is then pumped through a pump and piping system 102 and an optional meter 103 into the main transmission water pipe 104 used to convey water into the underground aquifer. The pumping system 102 may include automatic sulfide injection controls which are tied to continuous or periodic sampling of the input water quality.

In order to correct the pH of the water, an acid compound such as hydrochloric acid (or others described above) is stored in liquid form in tanks or drums 105. It is then pumped through pump and piping system 106 and a meter 107. A pH sensor 108 on the water flow line 104 may be installed to monitor and adjust the sulfide and acid injection rates or ratio to the water flow rate via a standard chemical controller 111. A sampling point 109 on the system may be used to verify water quality and check for various quality issues, such as the amount of oxygen or the amount of sulfide in the water. The sampling may be done continuously through a port monitor 109 or it may be done periodically by taking samples back to a laboratory and manually adjusting the sulfide and acid pumping systems 102 and 106.

FIG. 2 provides a simplified sketch of a pumping system for an injection well. An optional sulfide treatment (removal) system 201 may be used to treat water withdrawn from the aquifer 204 as needed. Pumps are used to move the water flow 206 in and out of the aquifer 204 or natural underground storage aquifer. The pumping system that puts the water into the underground aquifer is commonly associated with a water treatment plant operation or a water distribution system. Systems based on gravity flow may also be employed. For many recharge applications the recharged water is never recovered or is recovered at a point distant from the recharge operation. The pump used to withdraw water is often located within the casing of the well. Typically, the target storage or recharge aquifer lies 100 or more feet below land surface though recharge systems to shallow aquifers are also common.

FIG. 3 provides a simplified sketch of typical embodiments of groundwater recharge systems which may utilize the method disclosed herein to resist trace metal mobilization and remove dissolved oxidants from water introduced into these systems. These include an infiltration gallery 301, infiltration trench 302, dry well 303, infiltration pits or spreading basins 304, or infiltration shaft 305. Other variations of these structures are also used such as stream channels, closed depression, or land application such as irrigation or other methods of applying water to surface or to permeable layers beneath the surface. The intent of all of these applications is to introduce the water to the unsaturated zone above the aquifer to induce the water to flow into the aquifer.

Various other recharge arrangements, known in the art, may be utilized with success, and include suction pumps, submersible pumps, wells, pits, trenches, and gravity flow structures. FIGS. 2 and 3 are illustrations of possible environments for utilization of the present method. The water recovered from storage may not need to pass through the sulfide treatment system 201 before being used.

Water aeration systems are known in the art. For example, U.S. Pat. No. 5,618,417 discloses a counter flow water aeration system which uses a turbo blower to deliver a higher volume of air to purify gas and/or iron laden water. The teachings can readily be adapted to provide oxygen in the water to react with any residual amount of sulfide compound remaining in the water. U.S. Pat. No. 5,618,417 is hereby incorporated by reference for all purposes. Various other ways are known to introduce oxygen into water and include creating waterfalls, cascading systems over rocks or obstacles, and open air tank agitation. Any or all of these methods can be employed to add oxygen to the water in order to react with any residual amount of sulfide compound remaining in the water prior to its use.

While various embodiments of the present method, which is the use of the sulfide ion to reduce or eliminate dissolved oxygen from water prior to introduction to the underground for recharge or storage in order to control the dissolution of negative valence sulfur minerals in the subsurface, have been described, the invention may be modified and adapted to various operational methods to those skilled in the art. Therefore, this invention is not limited to the description and figure shown herein, and includes all such embodiments, changes, and modifications that are encompassed by the scope of the claims. 

1. A method to remove dissolved oxidants from water introduced into a subsurface aquifer and to resist trace metal mobilization in said subsurface aquifers due to the dissolution of minerals containing sulfur comprising the steps of: providing a recharge conveyance system to allow said water to reach the aquifer; recharging said water to said aquifer while measuring the flow rate of said water being recharged; forming a treated recharged water by adding a measured amount of sulfide compound into said recharge water based on said flow rate and chemical composition of said recharge water; and storing said recharged water in said underground aquifer
 2. The method of claim 1, further comprising the step of: allowing said water to remain in said underground aquifer to increase the hydraulic head in the aquifer or for subsequent withdrawal
 3. The method according to claim 1, wherein said sulfide compound is a compound selected from the group consisting of Group Ia and Group IIa metal sulfides, hydrosulfides (bisulfides), and hydrogen sulfide.
 4. The method according to claim 1, wherein said sulfide compound is sodium hydrosulfide.
 5. The method according to claim 1, wherein said aquifer has strata containing arsenic and other trace metals.
 6. The method according to claim 1, wherein said the water is recharged into said aquifer increasing the hydraulic head pressure in said aquifer and changing the groundwater flow direction in said aquifer.
 7. The method according to claim 1, wherein said recharge conveyance is a recharge aquifer system.
 8. The method according of claim 1, wherein said recharge conveyance is selected from the group consisting of a well, recharge basin, infiltration gallery, infiltration trench, infiltration pit, and infiltration shaft.
 9. The method according to claim 1, wherein said water reaches said aquifer via gravity flow or applied head pressure.
 10. The method according to claim 1, wherein the water quality of said aquifer is controlled by changing groundwater flow patterns laterally or vertically within or to said aquifer.
 11. The method of claim 1, wherein said introduction of said water limits migration of water containing undesirable chemical constituents within a USDW.
 12. The method according to claim 1, wherein said introduction of said water limits the introduction of salt into a USDW
 13. The method according to claim 1, wherein said aquifer is a USDW.
 14. The method according to claim 1, wherein said sulfur is in a formal negative oxidation state.
 15. The method according to claim 14, wherein said mineral is pyrite. 